Electrophotographic processes involve the transfer of an ink or toner image to a recording medium where the image is fixed. In typical electrophotographic processes, an electrostatic latent image is developed with toner and transferred to a recording medium, such as paper. The transferred toner image is fixed to the recording medium using known processes, such as by heating or fusing. Fixation of a toner image by heating typically involves the passing of a sheet of paper or other substrate containing toner particles through one or more fusing rollers. The heat applied by the fusing roller to the toner particles on the paper fixes the toner to the paper. The fixing of toner to paper or other substrates is well known.
One of the problems associated with the fixing of toner is the undesirable transfer of toner to the fusing roller during the fixing process. This undesirable transfer is sometimes referred to as “off setting” and involves the transfer of toner from the substrate to the fusing roller. The transfer of toner to the fusing roller contaminates the fusing roller, which results in the unwanted transfer of toner from the fusing roller onto subsequent substrates passing by the fusing roller. This transfer tends to produce ghost images or unwanted toner marks on subsequent substrates.
The off setting of toner to the fusing roller may be caused by a number of different factors. In some instances, toner is transferred to a fusing roller due to the cold offset or fixing temperature of the toner. The cold offset temperature is the temperature at which the toner begins to be fused. Typically, toners having lower viscosities have lower cold offset temperatures. In other instances, toner is transferred to the fusing roller because the toner loses its cohesive strength, producing a melted mass of toner that sticks to the fusing roller. The temperature at which this phenomenon occurs is known as the hot offset point or temperature. The hot offset temperature is related to the melt elasticity of the toner. Toners having lower melt elasticities exhibit lower hot offset temperatures whereas toners with higher melt elasticities have higher hot offset temperatures.
The difference between the hot offset temperature and the cold offset temperature is sometimes called the offset latitude, or a temperature range within which a fusing roller must operate in order to prevent off setting. In many toner fixing applications, the larger the offset latitude of the toner the better because a larger offset latitude provides a greater operational temperature range for the fusing rollers. Furthermore, a reduction in the cold offset temperature allows a decrease in the operational temperature of the fusing roller, which decreases the temperature in an elctrophotographic apparatus, prolonging its life.
It is therefore desirable to develop toner compositions having greater offset latitudes. This can be accomplished by developing toner compositions having lower cold offset temperatures and higher hot offset temperatures.